Measurement of the concentration of light absorbing analytes in human tissue has many clinical uses. For example, cerebral oxygenation can be determined by analyzing concentrations of oxyhemoglobin and deoxyhemoglobin in the brain. Cerebral oxygenation measurements have wide clinical usage including monitoring of high risk pregnancy, premature infants, monitoring of psychological or neurological condition of a patient, monitoring of cognitive state, and others.
One device for measuring the concentration of light absorbing analytes in human tissue is pulse oximetry. Pulse oximeters measure hemoglobin saturation, the ratio of oxygen actually carried by hemoglobin to the oxygen-carrying capacity of hemoglobin. Pulse oximeter saturation readings are useful as a noninvasive, continuous alternative to blood samples. One example of a pulse oximeter with multiple optical sources is disclosed in U.S. Pat. No. 5,902,235, entitled xe2x80x9cOptical cerebral oximeterxe2x80x9d and issued to Lewis. The pulse oximeter disclosed by Lewis has light-emitting diodes (LED) that produce two different wavelengths of light, 660 nm and 890-950 nm, and a common detector.
Pulse oximeters have several disadvantages. For example, pulse oximeters are not capable of measuring path length so short path lengths of less than 1 cm, typically through the finger or earlobe, are used for measurement. Tissue is alternately irradiated by the two LED sources and absorbance is measured on a xe2x80x9cpulsexe2x80x9d, the systolic portion of the heartbeat. The ratio of the two absorbances is multiplied by an experimentally obtained constant to determine saturation. Pulse oximeters generally have an experimentally determined table stored in memory that relates the ratio of the two absorbances to blood oxygen saturation.
Pulse oximeters can be used to detect fetal hypoxia. Hypoxia occurs when the oxygen supply to the brain is inadequate for normal cellular function. Hypoxia can result in brain damage and/or death of the fetus. Fetal hypoxia can result in the uterus if the umbilical cord wraps around a fetus"" neck, thereby restricting blood flow to the head. A high-risk fetus typically lacks cerebral blood pressure regulation mechanisms that are found in adults and normal fetuses. Contractions can cause cerebral hemorrhage that leads to hypoxia in high-risk fetuses.
Current technology includes two instruments that attempt to detect fetal hypoxia. Neither instrument produces reliable measurements. A first instrument is a Doppler ultrasound instrument that records the fetal heartbeat through the mother""s abdominal wall. Cardiac accelerations and decelerations on the recording are visually analyzed to determine whether the fetus is in a distress condition. A practitioner using a Doppler ultrasound instrument frequently fails to detect actual hypoxia and falsely detects the hypoxia condition when not present because the parameter sensed is fetal heartbeat rather than the more-efficacious parameter of fetal cerebral oxygenation. A second instrument is a pulse oximeter that is inserted into the uterus in contact with the fetal cheek. Pulse oximeters are inaccurate at common fetal saturation levels in the range from 35% to 50%.
Cerebral blood flow (CBF) is the amount of blood passing a volume of brain tissue. One system for measuring cerebral blood flow is described in U.S. Pat. No. 5,251,632, entitled xe2x80x9cTissue oxygenation measurement systemxe2x80x9d, and issued to Delpy. Delpy describes a system that uses near infrared spectrophotometry in combination with pulse oximetry to measure cerebral blood flow. The Delpy system has several components including a near infrared spectrophotometer and a pulse oximeter, in addition to a computer, a mixer, and a ventilator. The Delpy system uses the ventilator and mixer to generate a 5-10% step change in inspired oxygen and measures the change of oxyhemoglobin (HbO2) concentration induced by the oxygen infusion. The oxygen step is used as a tracer in the Fick method of determining flow. The computer calculates cerebral blood flow by dividing the rate of accumulation (dQ/dt) of oxyhemoglobin by the difference between the arrival rate and the departure rate of oxyhemoglobin. The amount of oxyhemoglobin in cerebral tissue is measured by the near infrared spectrophotometry. The difference between oxygen arrival and departure rate is the saturation of blood in the infant earlobe as measured by the pulse oximeter.
The Delpy technique for measuring cerebral blood flow has several disadvantages. The technique suffers from inaccuracies due to the measurement of Hb and HbO2 at different parts of the body and using different measurement methods. Multiple devices, including multiple sensors, a ventilator, a mixer, and a computer, must operate in cooperation with precise timing, introducing the possibility of error for each device. The Delpy technique has high complexity, requiring synchronization of multiple different medical devices with a computer. The Delpy system cannot be operated without specially trained staff and software, increasing complexity and cost. In addition, the Delpy technique introduces risk in varying the inspired oxygen concentration of critically ill pre-term infants and can only be used on patients who are mechanically ventilated.
Another clinical use of light-absorbing analyte measurement is diagnosis of acute lung injury involving any form of acute respiratory insufficiency. U.S. Pat. No. 5,679,532, entitled xe2x80x9cSerum Ferritin as a Predictor of the Acute Respiratory Distress Syndrome (ARDS),xe2x80x9d and issued to Repine, describes a method for determining the potential to develop ARDS, a severe subset of acute lung injury, in an at-risk patient. Repine discloses a technique for determining the patient""s serum concentration of ferritin in a blood sample and determining ARDS development potential from the serum concentration of ferritin. The disadvantage of the Repine system is that a blood sample must be taken.
Another clinical use of light-absorbing analyte measurement is determination of arterial blood gas concentrations. An arterial blood sample is drawn from the patient and parameters having clinical value are measured with laboratory instruments including pH, and the partial pressure of oxygen and the partial pressure of carbon dioxide. Clinicians use arterial blood gas measurements to adjust inspired oxygen concentration and respiratory rate of ventilated patients to assure adequate tissue oxygenation.
Near infrared spectrophotometry noninvasively and accurately measures the concentration of light absorbing analytes in human tissue.
In accordance with aspects of the present invention, an optical sensor includes an optical source capable of being positioned on a tissue and emitting near infrared light into the tissue at a plurality of selected wavelengths, and a photodetector capable of detecting reflected light from the tissue. The photodetector is positioned on the tissue removed from the optical source but sufficiently close in proximity to the optical source to contact the same general tissue. A high frequency oscillator is directly coupled to the source. The sensor is coupled to a radio frequency (RF) signal processor that is capable of detecting baseband modulation components from an RF carrier which makes use of the high frequency source oscillator. Examples of RF signal processors are direct conversion receivers, tuned RF receivers, superheterodyne receivers with either synthesized or variable frequency reference oscillators, and the like. The magnitude and phase of the baseband detector furnishes an estimate of the optical path length as well as the absorption. In addition, the source may be wavelength modulated by current control and/or by temperature cooler and a power supply to generate close-proximity optical wavelengths. The wavelength modulation shift in conjunction with additional baseband signal processing provides additional normalization to reduce scattering errors.
In accordance with aspects of the present invention, an apparatus includes (a) a single optical source capable of emitting near infrared light into the tissue at a plurality of selected wavelengths, (b) a detector capable of detecting reflected light in response to emission by the optical source, (c) a signal activator coupled to the optical source and capable of activating and modulating the optical source emitting the selected wavelengths in a range of wavelengths within one percent of a first nominal wavelength, and (d) an analyzer coupled to the detector and capable of analyzing changes in modulation intensity and phase between light emitted into the tissue and light reflected from the tissue to determine xcexca, the absorbance of the tissue, according to equation:       μ    a    =                              ln          ⁢                      xe2x80x83                    ⁢          10                                      -            2                    ⁢                      xe2x80x83                    ⁢          c                    ⁢              (                                            ⅆ              A                                      ⅆ                              μ                a                                                                        ⅆ              θ                                      ⅆ                              μ                a                                                    )              =                            ln          ⁢                      xe2x80x83                    ⁢          10                                      -            2                    ⁢                      xe2x80x83                    ⁢          c                    ⁢              (                              Δ            ⁢                          xe2x80x83                        ⁢            A                                Δ            ⁢                          xe2x80x83                        ⁢            θ                          )            
where dA/dxcexca is modulation amplitude difference and dxcex8/dxcexca is the modulation phase difference between two slightly shifted wavelengths in the selected range of wavelengths, and c is the speed of light.
Further in accordance with various aspects of the invention, the signal activator is capable of activating and modulating the optical source to emit the selected wavelengths in a range of wavelengths within one percent of the first nominal wavelength, emit the selected wavelengths in a range of wavelengths within one percent of a second nominal wavelength, and emit the selected wavelengths in a range of wavelengths within one percent of a third nominal wavelength.
In accordance with other aspects of the invention, a method of sensing a parameter includes emitting near infrared light into the tissue at a plurality of selected wavelengths, and detecting reflected light from the tissue at a distance removed from the emission but sufficiently close in proximity to contact a same general tissue. The method further includes activating emission of the near infrared light to emit a plurality of wavelengths that are selected to increase amplitude and slope of absorbency of a compound of interest within the tissue.